Bluetooth and Short Range Wireless Device Measurement

Short range wireless family includes Bluetooth, Zigbee, Z-wave, NFC, etc., standards. Standard group normally defines a suite of certificate tests that devices need to pass to get certificated. Here we use Bluetooth as an example to go through some of these tests and MORE importantly what could be the causes and possible solutions in a typical system.

A Bluetooth device can operate in different modes:

• normal mode

• transmitter (Tx) test mode

• loopback test mode

Normal mode consists of having a standard Bluetooth communication. For instance, when the tester is acting as a master and the Bluetooth device as a slave, in normal mode thetester will send some POLL packets and the device will confirm the reception of these packets by sending back a NULL packet.

In Test mode, the Bluetooth device is operating in a specific state. In Loopback test mode, the Bluetooth device (slave) is asked to decode the packets sent by the tester (master) and send back the payload using the same packet type. While in Transmitter test mode, the Bluetooth device is simply asked to transmit a type of packet according to specific instructions sent by the tester (master) via POLL packets.

The implementation of Test mode in Bluetooth devices is mandatory. To allow the tester (master) to put the device (slave) into Test mode, the host device will need to send a special command (LMP command) in order to prepare the device to enter Test mode. The control could be performed either by protocol sent over an RF connection or by direct digital control of the device. The latter is through the HCI, Host Control Interface, of a Bluetooth device, which is most likely on a UART link.

The following is a summary list of the conditions under which the transmitter tests need to be performed.

Output Power

The output power test verifies the maximum peak and average RF-output power, in dBm or mW, of the DUT. The output power measurement is conducted in the time domain. Due to the burst nature of the Bluetooth signal, a triggered measurement is setup based on the rising edge of the burst. The tester calculates the average power (Pav) over at least 20% to 80% of the burst and also records the highest power (Ppk) in the trace. Below is the Power vs. Time plot for DH5 packet.

We need to do OP sweep to show how output power varies over the whole Bluetooth frequency band and also over the temperatures. Most likely OP will vary over band and temp and we need to determine if it is acceptable. If the variation over temp is beyond limit, design may need to change to add